We have developed a two-electrode method for localizing the sites of quantal transmitter release at the frog neuromuscular junction. We will use this method to study the sites of quantal release during simulation and facilitation. We are especially interested in testing the idea that facilitation may result from conduction further out of the nerve terminals. We also plan to test the possible effects of Mg2 ion on facilitation. This is based on the evidence that elevated intracellular Mg2 ion promotes spontaneous quantal release and that Mg2 ion enters the terminals during stimulation. BIBLIOGRAPHIC REFERENCES: Van der Kloot, W. (1977). Variables affecting transmission at the neuromuscular junction. In "Functional Electrical Stimulation." Ed. J.B. Reswick and F.T. Hambricht. Marcell Dekker, (in press). Kita, H. and W. Van der Kloot (1977). Time course and magnitude of effects of changes in tonicity on acetylcholine release at frog neuromuscular junction. J. Neurophysiol. 40: 212:224.